Holding cabinet with closed-loop humidity control system and method for controlling humidity in a holding cabinet

ABSTRACT

A holding cabinet with closed-loop humidity control system and method for controlling humidity in a holding cabinet are disclosed. According to an embodiment, the method comprises determining a relative humidity set point; activating a heater in a fluid pan; determining if a fluid is present in the fluid pan; measuring the relative humidity in the cabinet; and maintaining the relative humidity within a predetermined range. According to another embodiment, a holding cabinet with a closed-loop humidity control system includes a holding cabinet; an air temperature probe for measuring an air temperature within the holding cabinet; a humidity sensor for measuring a humidity within the holding cabinet; a heater for heating air within the holding cabinet to a predetermined temperature; an air fan for circulating the air and for introducing air from outside the holding cabinet; and a water pan for holding water within the holding cabinet.

[0001] The present application claims priority from U.S. ProvisionalPatent Application No. 60/217,707, entitled “Holding Cabinet withClosed-Loop Humidity Control System and Method for Controlling Humidityin a Holding Cabinet,” filed Jul. 12, 2000, and U.S. Provisional PatentApplication No. 60/156,449, entitled “Holding Cabinet with Closed-LoopHumidity Control System and Method for Controlling Humidity in a HoldingCabinet,” filed Sep. 28, 1999, which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a holding cabinet, whichprovides a more consistent and accurate holding environment for foodproducts. In particular, the invention relates to a holding cabinet,which provides a more consistent and accurate holding environment forfood products by providing closed-loop control of the humidity withinthe cabinet as a controlled process variable.

[0004] 2. Description of Related Art

[0005] With the increasing popularity of “fast food” establishmentswhere food is precooked for later sale, there is a demand for foodholding devices that maintain food at a substantially uniformtemperature for selected periods of time while preserving the taste,moisture content, texture and quality of the food. Further, in otherapplications, it is desirable to be able to restore food, particularlybaked goods, to acceptable quality after long storage periods.

[0006] In many instances, storage of “fast foods” is particularlydifficult because beat loss, bacteria growth and moisture lossexperience by the food at storage conditions provided by prior artdevices, particularly where the food is to be stored warm, contribute torapid deterioration of the food.

[0007] More particularly, it has been found that air circulationcharacteristics and improper storage temperature contributesignificantly to bacteria growth and excessive loss of moisture whichleads to food shrinkage, so that in improper storage atmosphere the fooddeteriorates after only a short period of time and loses its tenderness,appetizing taste, and appearance.

[0008] It has also been found that even where food is stored underfavorable conditions in an enclosure, the food deteriorates at a ratedependent on the time the door to the enclosure is opened so the storagechamber is exposed to the ambient atmosphere.

[0009] Additionally, it is known that in storage of some foods, such asfried chicken or fish, where a crust is provided, it is particularlydesirable to maintain the crispness of the crust while minimizing themoisture loss from the underlying meat. Storage of such foods tends toinvolve the satisfaction of seemingly mutually exclusive conditions, tohold the crispness of the crust by maintaining low moisture content inthe crust while minimizing moisture loss from the food. In such foods,excessive moisture-loss results in shrinkage and loss of tenderness andadversely affects the texture of the meat. This may be prevented bycontrolling the temperature and humidity of the storage atmosphere. Theproblem is to prevent moisture flow from the underlying food to thecrust while holding the crust in low moisture content.

[0010] There are presently numerous cabinets for holding food productsor other items in a temperature and humidity-controlled state. Thesecabinets, however, suffer from a common shortcoming. When the cabinetsare opened to insert additional food products or other items or toremove such products or items from the cabinets, heat and humidity arelost. Unless the lost heat and humidity is restored, the items stored inthe cabinets may cool or dry out, or both.

[0011] Proofing and holding are distinct food preparation processes.Proofing is a process generally applied to yeast bread products, inwhich the yeast grows and the bread rises due to yeast growth byproducts. Holding, however, is a process during which foodcharacteristics are maintained, e.g., the temperature, moisture content,texture, and color of the food remain unchanged. Thus, in proofing, foodproduct characteristics change, while in holding, those characteristicsremain the same.

[0012] In terms of process parameters, proofing may be distinguishedfrom holding mainly by lower process temperatures. Humidity may begreater than about 80% RH, but the selected humidity may vary widelydepending on the particular bread product to be proofed. Nevertheless,proofing temperatures are generally lower than holding temperatures.High proofing temperatures might inhibit yeast growth. However, highholding temperatures are desirable because such temperatures maysuppress the growth of bacteria, molds, and the like and may increasethe holding time for food products.

[0013] Previously, various methods and devices have been developed toattempt to maintain heat and humidity. For example, pans of water havebeen placed in the cabinets and allowed to evaporate naturally in anattempt to maintain humidity. Despite its simplicity, this method hasnot been completely successful. Natural evaporation does not quicklycompensate for humidity losses. Further, while humidity naturallyincreases, items stored in the cabinets are subject to the drying effectof heat. Moreover, because natural evaporation is effected by thetemperature within the cabinet, the rate of humidity adjustment mayfluctuate with temperature changes, but humidity adjustments willprobably lag behind such temperature changes.

[0014] Systems have been developed by which the heat and humidity levelsof air within a cabinet are more closely controlled. Air may be heatedby passing it over, across, or through various types of heatingelements. Air may also be passed over, across, or through water in orderto raise the humidity of the air. Despite these improvements, knownsystems remain unable to precisely adjust for losses of heat or humiditydue to disruptions to the cabinet environment, such as opening andclosing the cabinet access, and adding or removing food products orother items.

[0015] Further, the addition of heating elements and humidity generatingmeans create additional problems. If heat or humidity rise too quickly,the air within the cabinets could become overheated or too moist. Suchuncontrolled fluctuations in heat and humidity may be detrimental tofood product or other items stored within the cabinets.

[0016] Cabinets commonly are equipped with thermostats in an attempt tocontrol the heat of the air circulating within the cabinets. Bycontrolling the air temperature, however, the humidity of the air alsomay be affected. Nevertheless, such controls alone do not provideadequate control of the humidity within the cabinet. Moreover, athermostat or manual potentiometer may not maintain temperature andhumidity within predetermined parameters. Generally, such devices onlycause the heating elements to heat the air when the air temperaturefalls below a set value.

SUMMARY OF THE INVENTION

[0017] A need has arisen for a holding cabinets for attainingclosed-loop humidity control by means of an effective humiditytransducer. Still a further need has arisen for a cabinet that may beused for both proofing and holding. It is a feature of such a cabinetthat its control system defaults to a generally higher temperatureassociated with a holding mode of operation. It is an advantage of thisdefault setting that the cabinet may inhibit the growth of bacteria infood products.

[0018] A holding cabinet with closed loop humidity control system andmethod for controlling humidity in a holding cabinet are disclosed.According to an embodiment, the method comprises determining a relativehumidity set point; activating a heater in a fluid pan; determining if afluid is present in the fluid pan; measuring the relative humidity inthe cabinet; and maintaining the relative humidity within apredetermined range.

[0019] According to another embodiment, a holding cabinet with a closedloop humidity control system includes a holding cabinet; an airtemperature probe for measuring an air temperature within the holdingcabinet; a humidity sensor for measuring a humidity within the holdingcabinet; a heater for heating air within the holding cabinet to apredetermined temperature; a fan for circulating the air and forintroducing air from outside the holding cabinet; a slide vent andmotor; and a water pan for holding water within the holding cabinet.

[0020] According to another embodiment of the present invention, asystem for humidity measurement includes a humidity sensor; anoscillator circuit; and a microprocessor to measure oscillatorfrequency. According to another embodiment of the present invention, asystem for maintaining a relative humidity level in a cabinet includesmeans for determining a relative humidity set point; means foractivating a heater in a fluid pan; means for determining if a fluid ispresent in the fluid pan; means for measuring the relative humidity insaid cabinet; and means for maintaining the relative humidity within apredetermined range.

[0021] Other objects, features, and advantages will be understood bypersons skilled in the art from the following detailed description ofpreferred embodiments of the present invention, in view of theaccompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Embodiments of the invention are described below with referenceto the accompanying figures, which are provided by way of example only,and are not intended to limit the present invention.

[0023]FIG. 1 depicts a front view of the holding cabinet according to anembodiment of the present invention.

[0024]FIG. 2 depicts a side view of the holding cabinet according to anembodiment of the present invention.

[0025]FIG. 3 depicts a cross-sectional view of the holding cabinet ofthe present invention, along line III-III of FIG. 1.

[0026]FIG. 4 depicts a cross-sectional view of the holding cabinet ofthe present invention, along line IV-IV of FIG. 2.

[0027]FIG. 5 is a schematic depiction of the air and humid aircirculation within the holding cabinet according to an embodiment of thepresent invention.

[0028]FIG. 6 is a perspective view of a water pan cover and ringassembly according to an embodiment of the present invention;

[0029]FIG. 7 is a schematic depiction of the humidity generating pan andthe control and monitoring interconnections of the holding cabinetaccording to an embodiment of the present invention.

[0030]FIG. 8 depicts the circuitry of the humidity detection transduceraccording to an embodiment of the present invention.

[0031]FIGS. 9A and 9B are side and top views of a slide vent accordingto an embodiment of the present invention.

[0032]FIGS. 10A and 10B are schematic depictions of the slide vent andcabinet openings according to an embodiment of the present invention.

[0033]FIG. 11 is a flowchart of the process for vent operation accordingto an embodiment of the present invention.

[0034]FIG. 12 is a flowchart of the calibration process for the slidevent motor according to an embodiment of the present invention.

[0035]FIG. 13 is a depiction of the period of the slide vent accordingto an embodiment of the present invention.

[0036]FIG. 14A depicts a humidity regulation state diagram according toan embodiment of the present invention.

[0037]FIG. 14B is a graphical representation of the humidity controlprocess according to an embodiment of the present invention.

[0038]FIG. 15 is a flowchart of the process for increasing humidityaccording to an embodiment of the present invention.

[0039]FIG. 16 is a flowchart depicting the operation of the closed-loophumidity control system.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0040] Embodiments of the present invention and their technicaladvantages may be better understood by referring to FIGS. 1 though 13,like numerals referring to like and corresponding parts of the variousdrawings.

[0041] Referring to FIGS. 1 and 2, a front view of the holding cabinetand a side view of the holding cabinet according to an embodiment of thepresent invention are provided. Holding cabinet 100 has a front 102,back 104, and sides 106 and 108. Front 102 and back 104 may both have atleast one door with a corresponding locking mechanism 110. In theembodiment depicted in FIGS. 1 and 2, front 102 and back 104 each havetwo doors.

[0042] Module 114 is provided to house equipment used to control therelative humidity in cabinet 100. In an embodiment, holding cabinet 100may be provided with a plurality of wheels 112.

[0043] Referring to FIG. 3, a cross-sectional view of the holdingcabinet of the present invention, along line III-III of FIG. 1 isprovided. Referring to FIG. 4, a cross-sectional view of the holdingcabinet of the present invention, along line IV-IV of FIG. 2 is provided

[0044] Referring to FIG. 5, a schematic depiction of the air and humidair circulation within the holding cabinet according to one embodimentof the present invention is provided. Blower motor 708 is provided, asare heaters 706. In the embodiment shown, two heaters 706 are provided;other numbers and locations of heater 706 may also be used.

[0045] Water pan 316 is provided with water pan cover and ring assembly502, which is shown in detail in FIG. 6. Water pan cover and ringassembly 502 includes inner ring 520, outer ring 522, and cover 524.Steam exhaust ports 526 may be provided. In one embodiment, two exhaustports 526 are provided, at opposite sides of the rings.

[0046] Referring again to FIG. 5, water pan in water pan 316 is heatedby a water pan heater 506, which causes the water in water pan 316 tovaporize into steam 504. Inner and outer rings 520 and 522 of assembly502 concentrate heat generated by water pan heater 506, assisting in thevaporization.

[0047]FIG. 7 depicts a block diagram of system 700 according to anembodiment of the present invention. System 700 includes air temperatureprobe 702, which measures the temperature of the air in the holdingcabinet. Air temperature probe 702 may also be used to providetemperature compensation for humidity sensor 704 In one embodiment, airtemperature probe 702 may be part number DC32006A-3-18, manufactured byDurex Industries, Cary, Ill.

[0048] Humidity sensor 704 measures the relative humidity of the air inthe cabinet (H1). In an embodiment, humidity sensor 704 may be E&EElectronik Part No. EE00-FR3, manufactured by JLC International,Warminster, Pa. Air heater 706 heats the air in the cabinet to the setpoint specified by the user. In one embodiment, air heater 606 may bepart number U3-32-764-34, 500W, 1000 W, or 1500 W, manufactured byWatlow, Hannibal, Mo. Air fan 708 circulates heated air through thecabinet so that the entire cabinet volume is at the same temperature. Inone embodiment, air fan 708 may be part number SX-19695 (240V) orSX-20441 (208V), manufactured by Jakel, Highland, Ill.

[0049] Water pan 716 holds water to be boiled to create humidity. In oneembodiment, water pan heater 722 may be #-8-MSM22866-xxx, manufacturedby Minco, Minneapolis, Minn. In another embodiment, heating elements maybe screened onto water pan 716. Float switch 720 is provided todetermine the water level in water pan 716. In an embodiment, floatswitch 720 may control water flow into water pan 716 when the waterlevel is below a desired level. An water pan heater (RTD) temperaturesensor 723 is affixed to water pan heater 722. Alternatively, sensor 723may be integral with heater 722. Sensor 723 may measure the temperatureof heater 722 and input such measured temperature values to System 700.

[0050] Water pan heater temperature sensor 723 is linked to controlsystem 700 to ensure that water pan heater 722 remains off when eitherof at least two conditions occurs: first, when no water is in water pan716 or second, when float switch 720 fails. In normal operation, floatswitch 720 signals control system 700 that water pan 716 is empty, socontrol system 700 does not activate water pan heater 722. Nevertheless,line build-up, debris, or abuse may cause float switch 720 to fail inthe “full water pan” position. Water pan 716 and water pan heater 722may be quickly damaged if water pan heater 722 is activated while waterpan 716 is empty. Water pan heater temperature sensor 723 performs as abackup to float switch 720 to reduce or eliminate the risk of suchdamage to water pan 716 or water pan heater 722, or both.

[0051] Slide vent motor 730 controls the movement of the slide vent,which, in turn opens and closes the cabinet vent. Slide vent positionswitch 732 is provided to provide an indication of the status of thevent. In one embodiment, side vent position switch 732 may be partnumber KWABQACC, manufactured by Cherry Electrical Products, PleasantPrairie, Wis. Switch 732 may also be an switch, optical proximityswitch.

[0052] Process inputs and outputs connect to the process control asshown. Temperature sensor 798 is built into heater 722 and measures thewater pan temperature.

[0053] The cabinet air temperature is regulated with air temp sensor702, air heater 706 and air fan 708. The air temp regulation is obviousto those skilled in the art, and consists simply of regulating the airtemperature to the programmed set point. This may be a simplethermostatic (on/off) control with hysteresis, or may be a moresophisticated PID (proportional/integral/derivative) control algorithm.

[0054] Humidity may be regulated by 1) adding humidity when the cabinethumidity is below the humidity set point, and 2) decreasing humidity byintroducing outside ambient air to the cabinet, when the cabinethumidity is above the programmed set point. Thus, there are two separatesystems to regulate the humidity: a humidity generation system, and a“venting” system.

[0055] Referring to FIG. 8, humidity transducer circuit 800 according toone embodiment of the present invention is provided. Timer U1 forms anastable oscillator with output frequency, F_(O), set by capacitorsC_(x), C₁, and resistor R₁. Capacitors C₂ and C₃ bypass power supply.Capacitor C₁ blocks DC voltage to transducer C_(x), which is damaged byDC voltage. Resistor R₁ sets the frequency, F_(O). Resistor R₂ drainscharge from capacitor C₁ during power-down. Transducer C_(x) capacitancevaries with humidity. Microprocessor μP measures F_(O) period bycounting pulses (n₂) for {fraction (1/16)} second.

[0056] Example values for the elements in FIG. 8 are provided below:Element Value U1 LMC 555 C Timer R₁ 24.9 K R₂   5 M C₁ .039 μF, 50 V,1%, 100 PAM C₂  0.1 μF ceramic disk C₃   10 μF Tantalum C_(X) HumidityTransducer, E & E Electronik EE00-F123

[0057] The relative humidity percentage (% RH) may be determined by thefollowing equation:${\% \quad {RH}} = {419.734\left( {\frac{4343.287}{n_{2} + 360} - 1} \right)}$

[0058] Capacitance C_(X) also is affected by temperature, therefore, %RH is compensated for temperature with this equation:

% RH _(C)=[(T _(F)−140)(0.0016667)+1](% RH)

[0059] where:

[0060] T_(F) is air temperature in ° F. % RH_(C) is used to display andregulate humidity.

[0061] The systems of the present invention may implement a proofingmode of operation. As noted above, this invention may combine theproofing and holding functions in a single cabinet. For example, oninitiation of any power-up condition, a user interface, e.g., a display,for the control system may offer the user the opportunity to initiate a“Proof” option. The user may have a limited time window, e.g., ten (10)seconds, within which to accept this option. The user may accept theoption by activating a particular switch, e.g., a TEMP switch, or acombination of switches. If the option is not accepted during the timewindow, the control system initiates the hold (higher temperature) mode.However, if the option is accepted, the control system initiates theproof (lower temperature) mode.

[0062] The hold and proof modes are distinguished by the maximumallowable air temperature set point. For example, in the proof mode, themaximum allowable air temperature set point may be the minimum holdtemperature. Thus, if the minimum hold temperature were 150° F., themaximum proof temperature setpoint would be 150° F. Similarly, if theminimum hold temperature were 150° F., the maximum allowable hold modeair temperature set point might be 220° F., and the hold modetemperature range might be 150° F. to 220° F.

[0063] Referring to FIGS. 9A and 9B, side and top views of a slide ventaccording to an embodiment of the present invention are provided. Ingeneral, cabinet panel 902 is provided with slide panel 904. Bothcabinet panel 902 and slide panel 904 have at least one opening 906. Inone embodiment, openings 906 in cabinet panel 902 are fixed, whileopenings 902 in slide panel 904 slide relative to openings 906 incabinet panel 902. Gear motor 908 drives slide panel 904 linearly toopen or close openings 906 via lever arm 912 and slide pin 914. In oneembodiment, motor 908 is model number EB-5206, manufactured by CustomProducts, Inc., New Haven, Conn., or part number AB, manufactured byHurst Manufacturing Corporation, Princeton, Ind.

[0064] As slide panel 904 slides relative to cabinet panel 902, openings906 on slide panel 904 line up with openings 906 on cabinet panel 902,in effect opening a passage to the blower inlet and outlet (not shown).When slide panel 904 slides its full distance, openings 906 in cabinetpanel 902 are fully uncovered. At this point, slide panel 904 beginssliding in the opposite direction, and openings 906 in cabinet panel 902are covered, blocking access to the blower inlet and outlet (not shown).

[0065] Switch 916 is provided to indicate when vents 906 are fullyclosed. In another embodiment, switch 916 may be provided to indicatewhen vents 906 are fully opened. This variance may depend on theposition of switch 916 with respect to slide 904. Other arrangements maybe provided as desired. Switch 916 may be used during calibration todetermine the period of slide vent 904. This is discussed in greaterdetail, below.

[0066] Referring to FIGS. 10A and 10B, depictions of the slide vent inits closed and open positions are provided, respectively. In FIG. 10A,slide vent 904 is positioned such that air does not flow from theexterior of the cabinet into blower inlet 1010, and out of blowerexhaust 1012. When motor 908 is activated, however, slide vent 904 ismoved, shown in FIG. 10B, opens blower inlet 1010 and blower exhaust1012.

[0067] Referring to FIG. 11, a flowchart of the general operation of thecabinet is provided. In step 1102, the cabinet is powered up. This mayinvolved initializing cabinet components, which is known to one ofordinary skill in the art.

[0068] In step 1104, the vent motor is calibrated. This process isdescribed in greater detail in FIGS. 12 and 13, below.

[0069] Referring to FIG. 12, a flowchart of the slide vent motorcalibration process according to one embodiment of the present inventionis provided. The purpose of the calibration is to account for variationsin the actual time required to move the vent from one position toanother. Even though a synchronous AC motor may be used, the time forone revolution may vary because 1) the line frequency may be 50 Hz or 60Hz, and 2) friction and debris in the mechanism may slow the ventmovement.

[0070] In general, the control software needs to know the time for onecomplete revolution to be able to move the vent from the fully-opened tothe fully-closed position. The control knows when the vent isfully-closed, because a vent switch actuates at that position. Thus, ifthe actual period for the vent movement is T_(VENT), then the vent isfully open at time T_(VENT)/2. Also, the control may move the vent toother positions, such as 50% open area, by actuating the motor for sometime that is a fraction of T_(VENT). For example, to open the vent toabout 50% open area, the control activates the motor for aboutT_(VENT)/4, from either the fully-open or fully-closed position.

[0071] In one embodiment, although the vent open area is not a linearfunction of the vent motor actuation time, it provides a suitableapproximation, permitting the vent motor actuation time to be used toposition the slide vent. In another embodiment, different shapes for thevent holes may be used to provide a linear relationship between motoractuation time and vent open area.

[0072]FIG. 13 depicts the vent operation as far at the control isconcerned. As the motor turns and the vent actuates the vent switch, thevent switch is really actuated for some period of time, which may bereferred to as the “dwell time,” or T_(DWELL). The control may accountfor T_(DWELL) when calculating the time needed to actuate the motor toachieve a given vent position.

[0073] Referring again to FIG. 12, in one embodiment, the ventcalibration routine uses a timer that is always running, so there is noneed to start or stop the timer, just a need to reset it to find thedwell time and the period. In step 1202, there is a predetermined delay,during which timers and interrupts are synchronized. In one embodiment,this may be a one second delay; other delays may be used, as required.In another embodiment, this delay may be omitted.

[0074] In step 1204, after the timers and interrupts are synchronized,the vent motor is activated, causing the slide vent to move. The timeris cleared in step 1206, and, in step 1208, the control waits for afirst transition signal from vent switch. This signal indicates that thevent switch is being activated. If there is no switch signal within apredetermined time, an error message is presented to the user in step1210. This may be by a visual or audible signal, such as a CRT, a LED, abell, a chime, and the like. In an embodiment, a suitable message, suchas “Vent Stuck” is displayed for the user.

[0075] In one embodiment, the predetermined amount of time may be 48seconds. Other suitable lengths of time may be used as desired. Thistime may be selected based on, inter alia, the known general period ofthe vent. The time may also be selected to prevent damage to the motor.After the predetermined time is elapsed, the motor may be shut off.

[0076] If a signal is received from the vent switch, in step 1208, thetimer is cleared, and in step 1214, the control waits for a secondtransition signal from the vent switch, indicating that the vent switchis no longer actuated. Similar to above, if a predetermined time passeswithout a signal from the vent switch, the user may be notified in step1210. Once the second transition signal is received, in step 1216, thetimer is read, indicating the dwell time, or T_(DWELL). In step 1220,similar to steps 1208 and 1214, the control waits for a transitionsignal from the vent switch. Once a transition signal is received,indicating that the vent has completed its cycle, in step 1222, thetimer is read. This is T_(VENT).

[0077] In step 1224, the vent is moved to the fully-closed position. Asdiscussed above, this may be achieved by activating the motor forT_(VENT)/2.

[0078] The control may use the time required to move the vent to detectfaults in the vent system. If it takes longer than a predetermined timefor one complete revolution, the control assumes that the vent is stuck,or the motor has failed, and displays a fault message.

[0079] Referring again to FIG. 11, in step 1106, the control determinesif the vent position is within a predetermined tolerance of itsrequested position. In an embodiment, the vent position may be expressedas an opening percentage—from 100% open, to 0% open. In this step, it isdetermined if the actual position is within a predetermined window ofthe desired position. This may be about 10%, 5%, 2%, and the like. Inone embodiment, it is about 1%. If the vent is within this window, noadjustments are made.

[0080] If, in step 1108, it is determined that the vent is not withinthe predetermined window, the vent motor is activated for a determinedamount of time to move the vent to its desired position.

[0081] In step 1110, the device may be powered down. When this occurs,it is possible that humidity may condense on the humidity sensor as theair temperature within the cabinet drops. This may 1) damage thehumidity sensor, or 2) cause false humidity readings during operation.In order to compensate for this problem, in one embodiment, the deviceenters “purge” mode that is activated when the control switch is changedfrom “operate” to “standby” or “off.” In this mode, the air heater andthe water heater are turned off, and the fan is activated if thehumidity is greater than a predetermined level. The predeterminedhumidity level may be selected as a compromise between low humidity(much lower than 100%) and high ambient humidity that exists withinrestaurants or other operating environments. In one embodiment, thispercentage may be 80%.

[0082] When the fan is activated, air from outside the cabinet isinjected into the cabinet, for the most part, preventing the humidity inthe cabinet from exceeding the predetermined level. In general,controlling the humidity in the cabinet involves regulating the waterheat output and the vent motor output. The water heat output is usuallyturned on to increase humidity within the cabinet, while the vent isusually opened to reduce humidity within the cabinet.

[0083] According to an embodiment of the present invention, the humiditycontrol method consists of three states: Idle, Increase Humidity, andDecrease Humidity. Referring to FIG. 14A, a humidity regulation statediagram is provided. In the decrease humidity state, the vent is eitheropen 50% or 100%, depending on how far the actual humidity is above theset point. Other opening percentages may be used as desired. FIG. 14Bprovides a graphical representation of the humidity regulation.

[0084] In addition, the control levels of SP+9% RH and SP+7% just amountto a hysteresis band that switches between about 50% and about 100% ventopening.

[0085] In the Increase Humidity state, the net result of the flow chartlogic is to determine a duty cycle setting for the water heat output.The duty cycle is the number of {fraction (1/16)} second intervals, outof a period of 2 seconds that the water heat is on. For example, in aduty cycle of 25%, the heat is on for 0.5 seconds, which is 8 intervalsof {fraction (1/16)} second. Referring to FIG. 15, a flowchart depictingthe Increase Humidity logic according to one embodiment of the presentinvention is provided.

[0086] The humidity control is similar to PID control, but thederivative information is only used to update the integral term.

[0087] Blocks 1502 to 1508 set the water heat duty cycle when the actualhumidity is the same as the set point. If the temperature is below 125°F., the duty cycle is set to 25%. If the temperature is above 125° F.,the duty cycle is set to 31%. These cycles act to maintain the humiditynear the set point. A higher duty cycle is needed at highertemperatures. Blocks 1510 and 1512 set the duty cycle to 100% (full on)if the actual humidity is more than 3% RH below the humidity set point.This acts to bring the humidity back to the set point. Block 1514calculates the humidity error (humidity set point-actual humidity) andsaves it in a variable called hum_temp_byte.

[0088] Blocks 1516-1526 adjust the integral correction term I.E.L (whichstands for the code variable integral_error_level). The test in block1516 limits I.E.L. to values of 20 and 200. Block 1518 adds the humidityerror to I.E.L. Blocks 1520 -to 1526 add 5 to I.E.L. if the humidity isdecreasing, and subtract 20 from I.E.L. if the humidity is increasing.

[0089] The initialization of I.E.L. is not shown, but I.E.L. is set tozero whenever the Increase Humidity state is entered, or whenever themeasured humidity equals the set point.

[0090] The blocks in 1528 set a new variable, E.O. (for error_offset)from the value of I.E.L. just found. Note that a larger value of I.E.L.results in a larger value of E.O.

[0091] The blocks in 1530 find the duty-cycle on-time, called t(on).t(on) is a function of E.O. and the air temperature Ta. t(on) is justthe sum of a constant that depends on the air temperature and the valueof E.O.

[0092] Finally, block 1532 show that the actual duty cycle is calculatedfrom t(on)/31. The divisor is “31” because a 16 Hz clock is used for thewater heat output. The duty-cycle period is 2 seconds, but the clockactually counts from 0 to 31.

[0093] Referring to FIG. 16, a flow chart of the operation of aclosed-loop humidity control system is depicted. In this chart, T_(H) isthe water pan heater temperature measured by water pan heatertemperature sensor 723, and T_(UM) is the maximum allowable water pantemperature. A Float-Switch-Fault is true when float switch 720 hasfailed. Float switch 720 has failed when it fails to accurately detectsignificant changes in the water level in water pan 716.

[0094] Various operational conditions are detailed with respect to FIG.16. If water pan 716 is found empty during normal operations, floatswitch 720 will indicate allow water level (Step B) and a “low waterlevel” message is displayed (Step F). Water pan heater 722 then will bedisabled (Step I), and control system 700 will complete its operation(Step L).

[0095] Similarly, if water pan 716 is incorrectly found empty duringnormal operations, float switch 720 again will indicate a low waterlevel (Step B). However, control system 700 will inquire whetherT_(H)>T_(LIM) (Step C). If T_(H) T_(LIM), the Float-Switch-Fault is true(Step D), and water pan heater 722 is enabled (Step J). Control system700 then again completes its operation (Step L).

[0096] If a Float-Switch-Fault is detected, a low water level is againdetected (Step B) and control system 700 again will inquire whetherT_(H)>T_(LIM) (Step C). If T_(H)>T_(LIM), then water pan 716 is empty orlow on water and Float-Switch-Fault is true (Step E). The display maythen indicate “Float Switch Failed” and “Out of Water” or “Pan Empty”(Step G). Water pan heater 722 will be disabled (Step I), and controlsystem 700 will complete its operation (Step L).

[0097] While waiting for a Float-Switch-Fault to clear, Float switch 720will initially indicate that the water level in water pan 716 is low(Step B). Control system 700 then will inquire whether T_(H)>T_(LIM)(Step C). If T_(H) T_(LIM) then Float-Switch-Fault is true (Step D), andif whether T_(H)>(T_(LIM)−100° F.) or the reset delay timer is not setto zero (Step H), water pan heater 722 is disabled (Step I). Controlsystem 7800 then will complete its operation (Step L).

[0098] Once the Float-Switch-Fault has cleared, if Float switch 720indicates that the water level in water pan 716 is low (Step B), controlsystem 700 inquires whether T_(H)>T_(LIM) (Step C). If T_(H) T_(LIM),then Float-Switch-Fault is true (Step D), and control system 700inquires whether T_(H)>(T_(LIM)−100° F.) and whether the reset delaytimer is set to zero. (Step H). If both these conditions exist, theFloat-Switch-Fault is false (Step K), and water pan heater 722 isenabled (Step J). Control system 700 then will complete its operation(Step L).

[0099] Other embodiments of the invention will be apparent to theskilled in the art from a consideration of this specification orpractice of the invention disclosed herein. It is intended that thespecification and examples be considered as exemplary only, with thetrue scope and spirit of the invention being indicated by the followingclaims.

We claim:
 1. A method for maintaining a relative humidity level in acabinet, comprising: determining a relative humidity set point;activating a heater in a fluid pan; determining if a fluid is present insaid fluid pan; measuring said relative humidity in said cabinet; andmaintaining said relative humidity within a predetermined range.
 2. Themethod of claim 1, wherein said step of determining a relative humidityset point comprises: inputting said relative humidity set point.
 3. Themethod of claim 1, wherein said step of determining if a fluid ispresent in said fluid pan comprises: measuring a status of a floatswitch.
 4. The method of claim 1, wherein said step of maintaining saidrelative humidity within a predetermined range comprises: repeatedlymeasuring said relative humidity in said cabinet, and deactivating saidheater responsive to said relative humidity within a predetermined rangeof said relative humidity set point; and reactivating said heaterresponsive to said relative humidity falling outside of saidpredetermined range of said relative humidity set point.
 5. A holdingcabinet with a closed loop humidity control system comprising: a holdingcabinet; an air temperature probe for measuring an air temperaturewithin said holding cabinet; a humidity sensor for measuring a humiditywithin said holding cabinet; a heater for heating air within saidholding cabinet to a predetermined temperature; an air fan forcirculating said air and for introducing air from outside said holdingcabinet; and a water pan for holding water within said holding cabinet.6. The holding cabinet of claim 5, wherein said water pan comprises: atleast one heating element for heating said heating pan; and at least onefloat switch for determining a water level in said water pan.
 7. Asystem for humidity measurement comprising: a humidity sensor; and anastable oscillating circuit.
 8. A system for maintaining a relativehumidity level in a cabinet, comprising: means for determining arelative humidity set point; means for activating a heater in a fluidpan; means for determining if a fluid is present in said fluid pan;means for measuring said relative humidity in said cabinet; and meansfor maintaining said relative humidity within a predetermined range. 9.A method for maintaining a relative humidity level in a cabinet,comprising: initializing a cabinet system; calibrating a slide ventmotor; determining a desired relative humidity within the cabinet;determining a desired position for a slide vent; and positioning theslide vent in accordance with the determination.
 10. The method of claim9, wherein the step of calibrating a slide vent motor comprises:activating a slide vent motor; monitoring transition signals from aswitch; determining a dwell time; and determining a period of the slidevent.
 11. The method of claim 10, further comprising: providing an errormessage when no transition signals are received from the switch.
 12. Themethod of claim 9, wherein the step of positioning the slide ventcomprises: determining a present position of the slide vent; determiningwhether the current position is within a range of the desired position;actuating the motor responsive to a determination that the currentposition is not within a range of the desired position.
 13. The methodof claim 9, further comprising: purging the cabinet.
 14. The method ofclaim 13, wherein the step of purging the cabinet comprises: shuttingdown an air heater and a water heater; determining an actual relativehumidity level within the cabinet; activating a fan responsive to adetermination that the relative humidity is above a predeterminedhumidity level.